It looks as if you copied that field from my web page. Calculated Dates in Microsoft Word using Fields or vba
That field has line breaks to show the field without running off the page.
Spacing is crucial. In addition, it only works for +/- 28 days.

Do not try to type this field yourself! If you just wanted this field, you should download the document containing it. However, it will not give you a reliable date 30 days out.

Download the tutorial from Date Calculation Tutorial.
It is a Word document. Find the field you need and copy it into your document. Then modify the field to meet your needs.
You can also find it on Graham Mayor's site here: DateCalc.zip.